Conventionally, a scanning tunnel microscope (STM) and an atomic force microscope (AFM) are known as typical scanning probe microscopes (SPMs). The AFM includes a cantilever that has a probe at a free end thereof, a sensor that detects displacement of the cantilever, and a sample stage scanner. The sensor is typically a sensor of an optical lever type. The sample stage scanner typically includes a piezoelectric element and moves a sample in X, Y, and Z directions with respect to the cantilever.
In the AFM, the sample and the cantilever are scanned in the X and Y directions relatively to each other. In this case, displacement in the Z direction is feedback-controlled such that force acting on the cantilever and the sample is kept constant. The feedback control of the displacement in the Z direction is called Z scanning. It is possible to obtain a fine shape of a sample surface from the movement of the sample stage for keeping the force acting on the cantilever and the sample constant.
As measurement modes of the AFM, typically, an AC mode and a contact mode are known. In the AC mode, the cantilever is excited at a resonance frequency. Amplitude changes when the cantilever approaches the sample. Thus, the feedback control in the Z direction is performed such that the amplitude is fixed. Consequently, a distance between the cantilever and the sample is kept constant.
<Scanning Speed of the AFM>
In the conventional AFM, measurement is slow and time in the order of minute is required to pick up one image. This is because the feedback scanning is slow. The feedback scanning is to move the sample stage up and down to adjust a distance between the probe and the sample surface and keep the force acting on the cantilever probe and the sample constant. Since a long time is required to acquire an image, for example, it is difficult to observe movement of the sample.
Most of devices of the AFM are involved in a loop of the feedback scanning. The devices involved are the cantilever, the sensor, a sensor amplifier, a control circuit, a piezoelectric drive power supply, a sample stage scanner, and the like. Among these devices, usually, the sample stage scanner is a slowest device. Therefore, an increase in speed of the scanner is essential to increase imaging speed of the AFM.
However, in the conventional AFM, a limit of the increase in speed of the scanner is low. More specifically, in the conventional AFM, the scanner includes a piezoelectric element having a macro size. The sample stage is scanned in the three-dimensional directions of X, Y, and Z by the scanner. The feedback scanning is Z direction scanning and higher in speed compared with XY direction scanning. This Z direction scanning needs to be performed in a domain lower than a resonance frequency of the piezoelectric element. Therefore, in order to increase the scanning speed, it is necessary to increase the resonance frequency of the piezoelectric element in the Z direction scanning. However, since the piezoelectric element has a macroscopic size compared with the cantilever and the like, the resonance frequency of the piezoelectric element is low. This is a factor that prevents the increase in speed of the scanner.
In the feedback scanning, the cantilever may be moved in the Z direction instead of moving the sample stage in the Z direction. Thus, by introducing a piezoelectric thin film into the cantilever using the MEMS technology, a cantilever having a self-actuation function is developed. In this case, since the cantilever is extremely small compared with the sample stage scanner, it is possible to easily increase the resonance frequency. Therefore, it is easy to increase speed of the feedback scanning. However, since the cantilever has the self-actuation function, a structure of the cantilever is complicated and the cantilever is extremely hard. Therefore, The cantilever of the self-actuation type has a limit in that it is difficult to use the cantilever in measurement of fragile and soft samples such as biopolymer and synthetic polymer.
<Q Value Control for the Cantilever>
Incidentally, the cantilever is a sort of resonance system. As an amount representing sharpness of a resonance spectrum (a relation of displacement of the cantilever with respect to a frequency of an excitation force), there is a Q value (a Quality factor). As viscous resistance acting on the resonance system is smaller, the Q value is larger. Conversely, as the viscous resistance is larger, the Q value is smaller.
Response speed of the resonance system is represented by πf/Q. Here, f is a resonance frequency. As the Q value is larger, the response speed is lower. On the other hand, displacement sensitivity of the resonance system to an external force is higher as the Q value is larger.
Q-value control for artificially changing the Q value of the cantilever has already been proposed. The principle of the Q-value control is as described below. An equation of motion of the resonance system is described below.
[Numeral 1]m{umlaut over (x)}+γ{dot over (x)}+kx=F(t)
Displacement “x” is detected, the displacement x is subjected to time differentiation, and the time differentiation is multiplied by a coefficient “α” This value is added to or subtracted from an excitation force F(t) to obtain the following equation.
[Numeral 2]m{umlaut over (x)}+γ{dot over (x)}+kx=F(t)±a{dot over (x)}
In the case of addition (+), the viscous resistance decreases. As a result, the Q value increases. In the case of subtraction (−), the viscous resistance increases and the Q value decreases. In this way, it is possible to arbitrarily increase or decrease the Q value. Consequently, it is possible to adjust the response speed and the displacement sensitivity.
The Q-value control for the cantilever is control for applying an external force to the cantilever as indicated by the above equation. It is conceivable to apply the external force to the cantilever via some medium. However, since a delay in a phase occurs, even though the phase should be changed by degrees by the time differentiation, the phase is not precisely 90 degrees. Therefore, the control is not easy. When the external force is applied via a medium, mechanical elements around the cantilever also have resonance frequencies. This makes it difficult to accurately adjust a frequency of the external force to the resonance frequency of the cantilever.
Considering such matters, in the Q-value control, it is desired to directly apply the external force to the cantilever. Thus, conventionally, a ferromagnetic body is attached to the cantilever. Alternatively, the cantilever is coated with a ferromagnetic thin film. The external force is applied to the cantilever by an electromagnet. In the cantilever having the self-actuation function, it is also possible to directly apply the external force to the cantilever. However, in such a constitution, special work for the cantilever is necessary and mechanical characteristics (the resonance frequency and a spring constant) of the cantilever change.
A related art is disclosed in JP-A-2004-212078. In this related art, a laser beam is irradiated on a cantilever and the cantilever is excited by the laser beam.